It is known that for the separation of bodies from liquids, for example the separation of bodies from sewage or industrial process water, devices can be used which include a screen grid. The expression "bodies" here refers to small, though as a rule exceeding 0.1 mm. firm or soft bodies as well as relatively large ones such as e.g. stones, pieces of plastics, cardboard, wood etc. A liquid layer containing the bodies then flows along the upwards facing side of a screen grid oriented at an oblique angle to a vertical plane. The liquid successively passes through the screen grid and is fed to a receiving means which is located below the screen grid whilst the bodies remain on the upwards facing side of the screen grid and form so-called screenings. In most applications the screen grid has such a strong inclination--frequently more than 60.degree. towards the horizontal--that owing to the influence of the liquid flow and gravity the bulk of the screenings falls and/or slides down towards the lower part of the device where it is collected e.g. in a conveyor and removed. The screenings together with the accompanying liquid form a mass of a high liquid content. Before the mass is treated further it is often necessary, therefore, to reduce the liquid content which is done in special equipment often in conjunction with the device including the screen grid.
On using screen grids in the abovementioned context it is a general objective to combine the smallest possible size of screen apertures (so as to achieve optimum separation) with the wish that the screen apertures should not become clogged by the screenings separated. A clogging brings about a successively reduced flow area of the screen grid which after a time leads to the ceasing of the separating function of the grid. Consequently the device has to be disconnected and a cleaning of the screen surface performed at intervals which are too short to allow the efficiency necessary for a sound economy to be achieved. In accordance with a generally known method the cleaning is done by flushing the screen grid with water, often hot water.
The problem mentioned above becomes particularly serious when the liquid contains a high proportion of solid particles or bodies, as for example in the case of septic sludge from three-compartment septic tanks or waste water from food industries, slaughter-houses and paper/cellulose industries. A well-functioning separating equipment for such contaminated liquids is difficult to obtain except at high cost, and requires considerable space. To discharge septic sludge directly to municiple sewage treatment plants often creates problems--especially in the case of smaller units--since the purification process in the plant will be disturbed. Purification of process water often involves such high costs as to be unacceptable because of national or international competition which means that contaminants and polluted waste water are discharged in such large quantities that unacceptable environmental disturbances occur.
Wet screenings, e.g. slaughter-house offal, are difficult to handle and to transport and, moreover, it is often prohibited to dump such screenings. It is necessary, therefore, to dewater the screenings in spite of the high costs of such dewatering. For the dewatering special compactors are used which consist of special piston or screw presses or other special equipment. It has been found impossible to provide effective and functioning devices in spite of the investments for squeezing equipment exceeding many times the costs of the screening equipment itself.
When known and applied methods are used breakdowns frequently occur. Under these circumstances manpower is needed for supervising and cleaning of clogged filters which naturally gives rise to high costs. Various mechanical cleaning devices have been used for keeping the screen grid open. e.g. brushes. Furthermore, as mentioned already, flushing with hot water is also affected. Apart from such flushing not producing sufficiently good cleaning, large quantities of hot water are consumed which in itself involves high energy costs. As an example it may be mentioned that for cleaning a screen grid with 0.1 mm apertures and 1 m.sup.2 surface, each cleaning operation requires about 100 1 hot water. In the case of highly contaminated sewage the cleaning operation recurs relatively frequently e.g. twice per hour.
The disadvantages of the known and applied methods have led to other solutions being sought for keeping the screen grid free from clogging screenings. Thus, a technique is described, e.g. in the German patent publication No. DE 31 08 332 where the screen grid is cleaned with brush-like means which, whilst in contact with the screen grid, are displaced along the same and thereby loosen the attached screenings. To prevent the screenings accompanying the movement of the brush means from contacting the limiting edges of the screen grid and there to build up impenetrable banks, the brush means according to the patent are made to follow a specially programmed movement which entails the whole movement in direction towards the center of the device taking place chiefly with the brush means in contact with the screen grid, and the movement towards the limiting edges of the grid occurring with the brush means raised, at least in the region nearest the edges of the grid. It has been found, though, that from a point of view of reliability in operation the result is unsatisfactory, among other things, owing to the difficult medium wherein the equipment operates. Moreover, the equipment requires much space and is slow, since the mechanical construction is such that the equipment cannot cope with the difficult medium in the region of the screen grid. This means large space requirement because the drive unit has to be placed outside the actual screen structure and often in such a manner that the width dimension is increased by more than the entire length of stroke of the brush means. It is also known that reliable drive units for the reciprocating movement of cleaning means are not obtainable.
The problem described is especially aggravated when contaminants have to be separated from a strongly polluted liquid e.g. when freeing liquid from the septic sludge formed in well drains. To achieve this slotted screens with very narrow slots are used. e.g. of the order of magnitude of 1 mm. In the course of the separation, owing to the composition of the sludge and the small size of slots, very rapid clogging of the slots and consequent reduction of the flow area occurs. In order to achieve technically/economically realistic efficiencies, it is necessary therefore, in view of the relatively large volumes of e.g. septic sludge which have to be dealt with, to have access to operationally reliable and effective cleaning devices. Very high demands are made, therefore, on the function and reliability of the driving means in combination with the speed available for the moving of the cleaning means.